Adding a better GPU to that rig will definitely help. The Nvidia GT cards are quite weak and cannot handle any real gaming loads. The 750 TI is a pretty capable low power card. If I remember correctly, it uses roughly 70 Watts under load, and performs on par with an R7 260X which uses roughly 40 more Watts, but is cheaper and overclocks better. It should be noted that an R7 265 (Re branded and tweaked HD7850) will beat either of them quite readily, but uses even more power ( Roughly130 Watts?), and costs about $25 more in most cases. Any of those 3 cards will play most games at medium to high detail settings at 1600X900 resolution, maybe even 1920x1080 if some settings are lowered such as AA. Of the 3, the 265 is the clear winner for raw performance, but is the most power hungry and the largest. The 750 TI is the smallest and uses the least power, has strong performance, but will run into bottlenecks at higher resolutions and higher levels of AA due to its 128 bit memory interface. If you have a budget of $150, the R7 265 is the overall winner, if you want to spend a bit less, or build a Micro ATX rig in the future, the 750 TI is the winner. Keep in mind that these 3 cards are higher end budget cards, and there are cards much faster at around $200.
As far as the RAM and CPU are concerned, you should be good there for now, although you are better off running 2 sticks of RAM in dual channel (2 slots) rather than odd numbers in most cases. If you wanted to upgrade your RAM, you could get 2 4GB sticks, which of course can be re used in a new build later along with the GPU. That CPU is dated, but should be enough to handle a mid grade GPU with little or no real bottle necking. It is slightly slower than newer i3's such as the 3220, and a newer i5 such as a 3570k or 4670k will stomp it.
An SSD is an excellent upgrade to any system, and will make it run much faster and smoother in and around the O.S., but if you were to go that route, you would need to have a copy of windows to install on it so you could use it as a boot drive while using your regular hard drive for mass storage and games. You could also buy one to use as a cache drive for the hard drive you have now, if the SSD is sold as a cache drive it will usually come with the software required for it to work that way. The cache drive method works quite well, but buying one to use as a boot drive is a better option.
If you do a GPU upgrade, a new power supply is highly recommended, even with a 750 TI. Most OEM builders only use power supplies that are only powerful enough to run what the computer came with, and are rarely more than 350-400 Watts unless the rig is a prebuilt gaming rig from Origin PC or Cyberpower PC. You will be able to add RAM or another hard drive and it will power that, but anything more will likely kill it. On top of that, standard prebuilt PC power supplies are bulk units, and are usually not certified so they may even deliver less power than what they are supposed to. Not to mention that as they heat up, they are less efficient and may deliver less due to heat. For any of the 3 GPU's I listed, a Corsair CX 500 is plenty and is about $50. At times, the CX 600 will go on sale for $35 or so after a rebate. I've used the CX series for a few builds and have yet to have a complaint or a failed unit. It's worth spending the extra money to ensure the power supply doesn't light up and take other components with it. I've seen cheap OEM supplies actually start on fire when they are loaded too much, at the least, it will probably die and the PC will randomly shut down to protect itself.
And lastly, if planning for a future ground up build, you are better off with AMD FX CPU's for a budget machine. The A series APU's with the built in GPU such as a Richland are nice for a daily use web browsing PC, budget gaming machine, or an HTPC build, but are not optimal for use with a discrete GPU such as the 750 TI. An FX 6300 will perform much better at roughly the same price point, and will sometimes go on sale for below $100.
That turned into a bit of an essay
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, but hopefully it was helpful!